1. Field of the Invention
The present invention relates in general to the field of integrated circuits and, more specifically, to an improved circuit for sensing and programming fuses in integrated circuits.
2. Description of the Related Art
Fusible links, sometimes referred to as programmable fuses or “efuses,” have found widespread use in integrated circuits. For example, programmable fuses can be used to store calibration information, encryption keys, “die ID” information and information for the configuration of optional features in the integrated circuit.
Large arrays of programmable fuses can also be used to make special purpose processors. For example, EPROM blocks in processors can be used to make power versus performance trade-offs by programming a mode bit. Another use for programmable fuses is to improve the yield of integrated circuit manufacturing processes. Designers often rely on redundant circuit components that can be made operable to replace defective components in an integrated circuit. When a defective component is detected, it is replaced with a redundant component by programming efuses in a fuse array.
In the examples discussed above, the fuse array must be read both during a testing procedure and during normal system operation. During these procedures, it is important to restrict the read current through an unblown fuse element in order to preserve its integrity. Since the process of programming or “blowing” a fuse occurs at a much higher voltage than the normal operation voltage, the control circuitry needs to operate robustly over a large voltage range. Furthermore, to calibrate the quality of the blown or unblown fuse, different sensing modes are required, thereby necessitating a dynamic mode-based threshold for the sensing scheme. A conventional voltage sensing scheme generally is not satisfactory given the constraints described above.
To ensure the quality of blown/unblown fuses, the fuses need to be read out during the test procedure (at a higher supply voltage) so that they can be re-programmed if necessary. However, at a higher supply voltage, the current through an unblown fuse can be large enough to partially program the fuse and increase the fuse resistance. This can make the fuse “stubborn” and unprogrammable. Also, such an effect is cumulative, i.e., it would depend on the number of times the fuse is read-out before programming it.
In view of the foregoing, there is a need for an improved sensing circuit for use in conjunction with programmable fuses used in integrated circuits. Such an improved circuit is provided by the method and apparatus of the present invention, as described hereinbelow.